Connecting an elongate member to another element may be accomplished in a variety of manners well familar to those skilled in the art. In those applications where the elongate member need never be detached from the other element, the connection between the two can take the form of a permanent bond such as a welded, riveted or interference-fit connection. However, when it is necessary that the elongate member be readily adjusted or detached relative to the other element, the choice of connection methods is restricted. For such applications, one of three classes of connectors is commonly employed. These are: interlocking connectors, such as threaded and bayonet couplings; clamp-type connectors, such as bolted flange coupling; and, wedge-type connectors, in which a tensile or compressive loading imposed on the elements being connected establishes some form of wedging action locking the two elements together.
One of the most demanding applications of adjustable connectors is the tether to platform connection in a tension leg offshore platform. A tension leg platform includes a buoyant platform attached by a set of elongate tubular tethers to a foundation on the seafloor. The location at which the platform is attached to each tether is selected such that the platform is maintained at a draft several meters greater than would be the case were the platform not restrained by the tethers. The resulting buoyant force of the platform imposes a high tensile loading on the tethers. This greatly restrains platform motion induced by waves, currents and winds. The tether to platform connectors must allow ready adjustability of the tether length such that the tension may be maintained at an optimum level in response to changing environmental conditions and varying platform buoyancy. These connectors must also be capable of withstanding the extremely high axial loads imposed by platform buoyancy. Further, because the axial load on the tethers includes a cyclical component resulting from wave action, it is important that the connectors be highly resistant to fatigue.
One tension leg platform design, described in an article entitled "Hutton-Tension goes on at World's First TLP", Offshore Engineer, August 1984, pp. 26-31, utilizes a threaded coupling to secure each tether to the buoyant platform. The upper several meters of each tether are threaded to receive a corresponding threaded collar. The threaded collar rests atop a mooring flat in the tension leg platform such that with the tether in tension the collar applies a downward force to the platform. Tether tension is adjusted by rotation of the collar in the appropriate direction. This design is disadvantageous in that the high loadings at the threaded interface between the collar and tether can cause the collar to seize in operation. Further, in applications in which a buoyant tubular tether is required, the large tether diameter introduces significant machining and assembly difficulties.
Many of the disadvantages of threaded connectors may be avoided through use of a wedge-type connector. However, existing wedge-type connectors have been thought unsuitable for use as tether couplings in tension leg platforms. Wedge-type connectors are used for releasably connecting a tubular element to a structure where the tubular element is under a high axial load. A typical example, adapted for connecting a tubular casing string to a riser pipe, is shown in U.S. Pat. No. 4,167,279, issued Sept. 11, 1979. Such connectors employ a series of arcuate slips arranged in an annular array intermediate a cylindrical support element and the elongate tubular element. Connectors of this type are not well suited for supporting tubular elements in situations in which the axial load is of the magnitude occurring in tension leg platform tethers. Casing slips operate by imposing a high, radially inward load on the supported tubular element. This radially inward load is directly proportional to the axial load on the tubular member. If the axial load is sufficiently great, the radially inward force imposed by the slips can become great enough to collapse the tubular member at the position at which it is supported.
It would be advantageous to provide a connector for securing an elongate member to another element wherein the point of connection on the elongate element is readily adjustable. It would be further advantageous to provide a connector for use in applications requiring high axial loading wherein the connector does not impose high radial loadings on the elongate element.